Just to take this a stage further, if you employ self employed people to do the job then you are effectively sub-contracting the work out, the main problem in this is that the sub contractor must be insured as he wont come under your general P/L insurance. so if there is an accident on site and the subby denies or does a runner you will not be insured, also if you are paying cash in hand and their is not a casual staff clause in your insurance then your not covered, this could lead to all sorts of litigation against yourself. In short, keep it legal, pay your taxes, and keep the industry professional, it is the cowboys who always lose their contracts and their reputation.
regards Dave ADL
PS if anyone has issues with tax or ni phone the inland revenue they will send an adviser to your workplace to go through all the inn and outs of the subject.